1. Field of the Invention
The present invention relates to multi-ply polyethylene-based separator systems for use between positive and negative plates in a silver-metal battery, particularly a silver-iron battery.
2. Description of Related Art
Silver-iron batteries are well known in the art, and are taught by Brown, in U.S. Pat. Nos. 4,078,125 and Buzzelli in 4,383,015. These patents teach the use of perforated silver sheet or expanded silver screen supports containing active silver material for positive plates, either sulfurized iron oxide negative plates according no the teaching of Jackovitz et al., U.S. Pat. No. 4,356,101, or sintered metallic iron negative plates. Brown and Buzzelli, in the above patents, both taught a multi-ply separator between positive and negative plates. The separator contained alternating porous and microporous sheets of polypropylene. One of the sheets, made of 60% to 90% porous, non-woven polypropylene, having 4 micron to 30 micron pores, was placed next to the silver electrode. The microporous polypropylene had pores of from about 0.05 micron to 3 micron diameter. Total separator thickness was generally about 0.050 to 0.075 cm.
The silver-iron battery is generally considered more stable than the silver-zinc battery. The silver-zinc battery has always presented major problems of internal electrical shorts due to zinc; dendritic growth from the negative plate through the separator system. The soluble silver in both silver-zinc and silver-iron systems has also caused some problems. One problem has been the tendency to form a silver conducting film on the separators, which could allow shorting. Both battery systems are quite expensive, and are usually restricted to applications where the energy density of the battery is critical to the total system mission. An example of such an application is a propulsion system power source for underseas vehicles.
A number of patents have issued on improved battery separator materials for use in silver batteries, most for use with silver-zinc couples. Langer et al., in U.S. Pat. Nos. 3,749,604 and 3,953,241, taught a porous, caustic resistant, polymeric support, such as polytetrafluorethylene (TEFLON) or the like, coated on at least one side with a polymeric matrix, such as polysulfone having pore diameters of from about 5 microns to 50 microns, containing inorganic filler particles. This separator was found useful for silver-zinc or silver-iron couples.
Moshtev et al., in U.S. Pat. No. 4,234,623, taught a five layer battery separator for alkali accumulator batteries. The separator contained, in order: inert, outside polyester layer; cellulose material, such as cotton, impregnated with methacrylic acid; central, irradiated, activated, low density polyethylene film, about 35 microns thick, graft polymerized with methacrylic acid, where there was a high degree of grafting, i.e, 80+%; cellulose material, such as cotton, impregnated with methacrylic acid; and inert, outside polyester layer. The minimum separator thickness, prior to any pressing for such a separator, was about 495 microns.
Nagamine et al., Japanese Patent Kokai No. 54-50829 (Application No. 52-116415), relates to separators for silver-zinc mercurate button cells. The separator contained, in order: outer cellophane film; porous, synthesized, high-molecular weight polyethylene, polypropylene, polytetrafluoroethylene, or polyester film; and outer cellophane film. Another embodiment of the separator contained one piece of cellophane film, and either one or two pieces of porous, synthesized, high-molecular weight polyethylene, polypropylene, or polytetrafluoroethylene film.
Nagamine et al. Japanese Patent Kokai No. 57-95069 (application No. 55-171763), relates to a laminated separator for silver-zinc button cells. The laminated separator contained, in order: irradiated, polyethylene film, graft polymerized with acrylic acid or meth-acrylic acid next to the silver anode material; cellophane film; irradiated polyethylene film, graft polymerized with acrylic acid or meth-acrylic acid; an outside single or double cellophane sheets. The prior art was characterized in Table 1 of this patent as cellophane film sandwiched by two pieces of graft polymerized polyethylene films with equal graft rates.
Adams et al., in U.S. Pat. No. 4,144,301, taught a deposited film or shaped envelope separator, for use in electrolytic cells. This separator contained a single sheet of irradiated, low density polyethylene, or polypropylene, graft polymerized with acrylic acid or meth-acrylic acid.
Many of these separator materials may be somewhat resistant to oxidation by divalent silver ions, many of them may also allow cellophane degradation by silver ions, and most would allow long term diffusion found to be subject to hydrolytic attack and to degrade an electrolyte at temperatures over 45.degree. C. Polypropylene cellophane combinations have been found to allow large scale silver mirror build-up, which over a long period time could cause shorts between any silver-metal battery couple. The separator can be the weakest component in a sophisticated battery, and generally is the primary life-limiting source for silver-iron batteries, since the iron electrode is stable, unlike zinc electrodes.
In silver-based cells, cellophane film is generally used in a sacrificial manner to trap migrating silver ions. In this process, the cellophane film is broken down. The breakdown of the cellophane film, combined with the general instability and solubility of cellophane in alkaline media, contributes to a chemical, and finally a mechanical, deterioration of the cellophane film. In silver cells using only cellophane film as the separator, the deterioration of the cellophane film permits pieces of cellophane to become mobile in the cell's electrolyte. In a worst case, the breakdown of the cellophane film may expose the positive and negative plates to one another, causing a hot short condition to develop. The typical remedy to this problem is to use from 5 to 7 wraps of cellophane to delay the overall breakdown of the cellophane film and extend the cellophane life.
In response to the problems associated with the prior art, Jackovitz et al. in U.S. Pat. No. 4,804,598, describe a separator system for silver-iron batteries, where the separator system contains at-least a layer of low density polyethylene, graft polymerized with an acrylic material, disposed next to the silver electrodes; and at least a layer of high density polyethylene, graft-co-polymerized with an acrylic material, disposed next to the iron electrodes. A layer of cellophane can also be included next to the low density polyethylene and a middle layer of low density polyethylene can be included between the cellophane and the high density polyethylene. The cross-linking of the polyethylene layers serves to increase the electrolytic resistance of the separator layer and therefore the resistance of the silver-iron battery cell. Because of this high resistance, this separator has limited applicability in low temperature high power applications. Consequently, there is a need for an improved separator system for use in silver-iron cells capable of improved low temperature high power performance.